The present invention relates to a facsimile transmission apparatus employing data compression.
In such a facsimile system, data from a scanner is run length encoded, modulated and transmitted to a remote receiver which demodulates and decodes the data for reproduction of an original document which was scanned by the transmitter. It is convenient to perform the run length encoding on a line by line basis corresponding to the orthogonal scanning pattern of the document.
In order to enable an input buffer memory at the receiver to have a small capacity and production cost, it is desirable to construct the system in such a manner that the time required for transmission of one line of coded or compressed data is shorter than the time required for decoding the data and reproducing the corresponding line. In this manner, the rate at which data is fed out of the input buffer at the receiver is faster than the rate at which data is fed in, and the buffer capacity may be small.
In order to ensure that the transmission time is longer than the operating time at the receiver, means are provided to ensure that each line of coded or compressed data comprises at least a predetermined number of bits. Since the code for scanning a perfectly blank linear area, for example, of the original document will consist of only a few bits, said means function to add enough fill bits, typically consisting of zeroes, to the code to increase the total number of bits to the predetermined number. A sync code is added at the end of the fill bits to indicate to the receiver that the end of the current line has been reached. In cases where the number of bits of coded data exceeds the predetermined number, no fill bits are necessary and only the sync code is added.
Prior art means for adding fill bits generally comprise a counter for counting the number of bits of the coded data, subtractor for subtracting the counted number of bits from the predetermined number and fill bit generator means for generating a number of fill bits equal to the output of the subtractor. Such circuitry is complicated and adds unnecessarily to the cost of the apparatus.